ORIGINS OF EMBRYONIC PATTERNS 673 



spermatozoon or its path in the egg has been regarded by some as deter- 

 mining the median plane in certain forms. The first cleavage plane is 

 definitely related to the median plane and so to the symmetry pattern 

 in some animals. It has been shown in earlier chapters that, at least in 

 certain organisms, symmetries and asymmetries show nonspecific differ- 

 ential susceptibility to external agents and can be altered, obliterated, 

 and determined in the same ways as polarity. These experimental results 

 suggest that polarity and symmetry patterns are not fundamentally dif- 

 ferent in nature. Moreover, the fact that obliteration or reversal of vari- 

 ous asymmetries by differential inhibition and recovery is not a specific 

 effect of a particular agent suggests that the differences of right and left 

 sides in the forms concerned results from a primarily quantitative physio- 

 logical difference. The physiological bases of axiate patterns appearing 

 at different developmental stages doubtless differ chemically, but as spa- 

 tial patterns they may be similar. For example, the polarity of a hydroid 

 tentacle doubtless differs chemically from the polarity of a hydroid plan- 

 ula, but they are both characterized by gradients which, as gradients, are 

 closely similar. In echinoderm development the pattern of differentiation 

 in relation to the polar axis differs from that in relation to ventrodorsality, 

 but at certain early developmental stages an apicoventral-basidorsal dif- 

 ferential or gradient is actually present (p. 134). 



Some lateral asymmetries appear indifferently on the right or left. We 

 regard the laterality of these as determined by incidental or chance fac- 

 tors, but there is still the question as to the origin and nature of the 

 asymmetry. The asymmetry of the opercula of serpulid annelids and of 

 chelae of various decapods is reversible experimentally by removal of the 

 dominant member of the pair (pp. 411-13). Some others are more or 

 less constant in their laterality for the species but may be reversed experi- 

 mentally or occasionally in nature — for example, the vertebrate visceral 

 asymmetries and various other experimental and natural cases of situs 

 inversus. The larval coelomic asymmetry of echinoderms may be either 

 completely obliterated or reversed experimentally. Certain spiral asym- 

 metries, such as the coiling of gasteropods, result from differential growth 

 on the two sides of the body, but some species usually or always coil in 

 one direction. The question how these asymmetries originate and how 

 their laterality is determined is still unanswered. To say that they are 

 inherited, that genetic factors are concerned in their laterality, though, 

 of course, true, throws no light on the physiology of their origin in de- 

 velopment. 



